1. Field of the Invention
The present invention generally relates to a photographic lens system and more specifically, but not exclusively, to an interchangeable lens system for a single lens reflex camera.
2. Description of the Related Art
A conventional image pickup apparatus, such as a photographic camera or a video camera, includes a macro-lens system or a micro-lens system (hereinafter collectively referred to as a “macro-lens system”) as a photographic lens system to be mainly used in photographing an object at a close distance.
The macro-lens system is designed to achieve increased optical performance in focusing on a close-distance object in comparison to other photographic lens systems such as general standard lens systems and telephoto lens systems.
The macro-lens system, can also be designed to be used for focusing on objects in a wide range extending from close to infinite distances.
In general, as the photographic magnification increases, when changing from an infinity focus to a close-up focus, various aberrations become greater, reducing the optical performance. Therefore, to improve these various aberrations, a photographic lens system using a movement referred to as “floating”, in which a plurality of lens units are moved independently of each other when focusing, has been discussed (refer to Japanese Patent Laid-Open No. 2-19814 (corresponding to U.S. Pat. No. 4,986,643) and Japanese Patent Laid-Open No. 2-285313 (corresponding to U.S. Pat. No. 5,007,720)).
Japanese Patent Laid-Open Nos. 2-19814 and 2-285313 discuss a lens system capable of close-up photography including a first lens unit having a positive refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power. This lens system, when changing the magnification from reduced power to increased power, carries out floating by fixing the third lens unit relative to the image surface and moving the first and second lens units towards the object side while changing the distance between the first and second lens units.
For example, an optical system, including a structure for compensation of blurring in a photographed image caused by vibration of the optical system (i.e., an image stabilizing function), has been discussed wherein the blurring is compensated for by moving a part of a lens unit constituting the optical system in a direction orthogonal to an optical axis (refer to Japanese Patent Laid-Open Nos. 2-35406, 8-136862, and 1-284823 (corresponding to U.S. Pat. Nos. 5,270,857, 6,124,972, and 5,040,881, respectively) and Japanese Patent Laid-Open No. 2001-272601).
In general, in order to make a macro-lens system that is capable of photographing in a wide range of distances, the extending distance (displacement) of the focusing lens unit should be large when focusing. If the displacement of the focusing lens unit is large, an actuator having a large driving torque will be used when using the macro-lens system in a camera having an automatic focusing function. Thus, high-speed automatic focusing tends to be difficult.
For a macro-lens system, fluctuation of the various aberrations, that occur when changing the magnification during photographing, can be great. Therefore, one should try and reduce this fluctuation.
By using a floating mechanism that moves a plurality of lens units during focusing, the fluctuation of the various aberrations due to focusing and the displacement of the lens units during focusing are both reduced.
Accordingly, employing a floating system for a macro-lens system not only reduces the fluctuation of the various aberrations that occur when changing the photographic magnification but also provides an efficient focusing mechanism for a camera having an automatic focus function.
However, including a vibration compensation mechanism into a lens system can be complicated, as described below. The vibration compensation function discussed in Japanese Patent Laid-Open No. 2-35406 is added to a zoom lens system suitable for a lens shutter camera. When this zoom lens system is used as an interchangeable lens system for a single lens reflex camera, the back-focus can be unsatisfactory.
An optical system discussed in Japanese Patent Laid-Open No. 1-284823 is capable of compensating for blurring of an image caused by vibration by using part of a first lens unit. However, since the diameter of the lens unit used for image stabilizing can be large compared to the diameters of other lens units, it can be difficult, for those particular embodiments, to mount the image stabilizing function to the optical system.
The recent digitization of single lens reflex cameras has lead to a reduction in the image size. Hence, an interchangeable lens system having a wide angle of view is useful. If the focal length of the lens system discussed in Japanese Patent Laid-Open No. 2001-272601 is reduced, the diameter of the lenses behind the aperture stop may have to be increased to obtain a sufficient amount of ambient light. Therefore, it could become difficult to add an image stabilizing function to the lens system.